1. Field of the Invention
The present invention generally relates to a laser diode driving method and a laser diode driving apparatus, which are used in an optical digital transmitter. More specifically, the present invention is directed to a laser diode driving method and a laser diode driving apparatus, in which deterioration of extinction ratio is prevented.
2. Description of the Related Art
To avoid the pattern effect caused by delay in laser light emission from a laser diode, a very small bias current which is lower than a threshold current value of the laser diode is generally supplied to the laser diode. As is known in this field, however, when the bias current exceeds the threshold value, the extinction ratio of the laser diode is rapidly deteriorated, resulting in a failure of signal transmission.
One of conventional bias current control examples is disclosed in, for instance, Japanese Laid-open Patent Application Disclosure (JP-A-Heisei 4-290483). This conventional bias current control example will now be described with reference to FIG. 1.
In FIG. 1, a time constant unit 34 produces a control signal Va in response to a mark ratio of a modulation signal "Vs", namely, a ratio of "1" to "0" contained in the signal. A shunting device 35 separates current "Io" supplied from a current source 36 into current Idc and current Ipo. Then, the current Idc is supplied to an adding unit 32, and the current Ipo is supplied to a driving unit 31. At this time, the shunting device 35 changes a shunt ratio of the current Idc to the current Ipo in response to the control signal Va.
That is, when the mark ratio of the modulation signal Vs is small, the shunt ratio is determined in such a manner that the current Idc is increased for a laser diode 33 subsequent to the adding unit 32. On the other hand, when the mark ratio is large, the shunting device 35 increases the ratio of the current Ipo to the current Idc. It should be noted that the following relationship should be satisfied: Ipo+Idc=Io, i.e., constant. The adding unit 32 adds the current Ip derived from the driving unit 31 to the current Idc derived from the shunting device 35 to produce a current ILd, and then supplies the current ILd to the laser diode 33. As described above, in the conventional example, the pattern effect can be prevented by detecting the mark ratio of the modulation signal Vs and then by changing the ratio of the current Idc to the current Ipo based on the detected mark ratio.
Next, another conventional laser diode driving circuit, which is described in Japanese Laid-open Patent Application Disclosure (JP-A-Heisei 3-24830), will be described with reference to FIG. 2. As represented in FIG. 2, a laser diode driving circuit 43 is composed of differential transistors 43-1 and 43-2, and constitutes a differential amplifier for driving a laser diode 46 in response to an input signal 41 together with a transistor 43-3. The pulse current control circuit 44 controls a drive current for the laser diode 46 to be increased/decreased in accordance with the light signal intensity from the laser diode 46 detected by a monitoring photodiode 47.
That is, the transistor 43-3 controls the drive current Ip in response to a signal 45 supplied from the pulse current control circuit 44. As described above, in this laser diode driving circuit 43, the laser diode 46 emits light in response to only the pulse current Ip without any bias current. Thus, the deterioration in the extinction ratio of the laser diode 46 can be prevented.
However, there is a problem in the above-explained laser diode driving circuit shown in FIG. 1. That is, when the mark ratio of the modulation signal is small, the current Idc is increased. Accordingly, there is a possibility that the extinction ratio will be deteriorated.
On the other hand, in another conventional laser diode driving circuit shown in FIG. 2, the laser diode is driven only by the pulse current without any DC bias current. As a result, when the data rate of an input signal becomes high, the pattern effect is caused, resulting in jitter phenomenon in a light output signal from the laser diode 46. As a consequence, the signal transmission quality is deteriorated.